Large relative aperture objective lens

ABSTRACT

A large relative aperture objective lens having three lens groups of positive power with said three lens groups being all moved forward to effect focusing from an infinitely distant object to close objects. The three lens groups, being named from front to rear, 1st, 2nd and 3rd lens groups successively, have the amounts of forward movement of the 1st, 2nd and 3rd lens groups made progressively smaller. Accordingly, good stability of aberration correction throughout the extended focusing range is achieved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to large relative aperture objective lenses, and, more particularly, to objective lenses of increased relative aperture while still permitting high grade imaging performance throughout an extended focusing range from infinity to very short object distances.

2. Description of the Prior Art

In the past, the photographic objectives called macro, or micro lenses which are adapted to be mainly used in closeup shooting and also capable of focusing up to infinity, when focused down to close object distances have suffered darkening of the effective F-number. This led to the production of a shade in the split prism area on the focusing screen. Therefore, the photographer had to turn to the surrounding or mat area for focusing purposes. Also, since the depth of field became shallow, it became very difficult to detect when the objective lens is focused to an optimum in-focus position. For this reason, there is an increasing demand for a larger relative aperture or fast objective lens.

However, since the range of variation of the image magnification is wide as compared with the normal photographic objective, aberrations vary to large degree with focusing. This aberration variation becomes prominent when the relative aperture is increased. Even in case, as usual, the start point for design on the image magnification is set in about 1/10 times, it is found that as the image magnification approaches 1/5 times or larger, the spherical aberration and curvature of the field becomes under-corrected, and asymmetric aberrations for off-axis rays are produced. Particularly regarding the outward coma in the intermediate zone to the marginal zone of the image format, for a subject of fine patterns, the image is largely blurred to impair the degree of sharpness, thus the photographer is obliged to use the objective with the diaphragm closed down. As it is also required to remove the distortion, it follows from these points that the symmetric lens type, for example, Gauss type with the inclusion of the modified Gauss type is suited for macro lenses of large relative aperture, and has found many uses.

With the ordinary Gauss type lens, however, in closeup shooting, the paraxial ray becomes stronger in divergence than when focused at infinity so that the height of incidence on the rear lens component becomes higher. The result of this is that the spherical aberration is largely under-corrected by the positive refractive power of the rear component. On the other hand, as regards the off-axis coma, because focusing down to shorter object distances is effected by moving the lens system forwards as a whole, the principal ray to the maximum image height makes a smaller angle with the optical axis so that the upper and lower portions of the image bearing beam are refracted more weakly by the rear lens component than when in shooting at an infinitely distant object, thereby outward coma is produced.

As a means for compensating for this drawback, it has already been known in the art, as disclosed in Japanese Laid Open Patent Sho Nos. 50-138823, 52-7723, 53-87728 and 53-10425 and U.S. Pat. Nos. 3,884,557 and 4,260,223, to change a certain air separation between lens members, as the entire lens system is moved forward, away from the image plane, with some advantage. However, this method, though being favorable to the closeup shooting of low image magnification, has drawbacks that, as the image magnification increases, the state of correction of the image aberrations cannot be maintained sufficiently acceptable. Further while the off-axis asymmetric aberrations are well corrected, the other various aberrations are somewhat sacrificed. The problem of achieving a large increase in the relative aperture while maintaining good stability of aberration correction throughout the extended focusing range has not been solved yet.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a photographic objective lens with means of relatively simple form making it possible to achieve a much-desired increase in the relative aperture while still permitting good correction of aberrations even at extraordinarily increased magnifications.

One of the features of the present invention is the arrangement of three lens groups each having a positive refractive power in such a way that, as the lens groups are numbered consecutively from front to rear, when focusing is effected down from infinity to shorter object distances, the three lens groups are moved axially forward, wherein the 2nd lens group is made to move faster than the 3rd lens group, and the 1st lens group is made to move faster than the 2nd lens group.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-a and 1-b, FIGS. 2-a and 2-b, and FIGS. 3-a and 3-b are lens block diagrams of embodiments of the present invention respectively.

FIGS. 4-a to d and 5-a to d are graphic representations of the various aberrations of the objective of FIG. 1.

FIGS. 6-a to d and 7-a to d are graphic representations of the various aberrations of the objective of FIG. 2.

FIGS. 8-a to d and 9-a to d are graphic representations of the various aberrations of the objective of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will next be described in connection with embodiments thereof by reference to the drawings.

FIGS. 1 to 3 in longitudinal sectional views illustrate embodiments 1 to 3 of objective lenses according to the present invention wherein I, II and III denote the 1st, 2nd and 3rd lens groups respectively with (a) representing an operative position of the lens when focused at infinity and (b) when at an image magnification of 0.85.

The objective lens of the invention is constructed from three lens groups of positive power and operates in such a manner that, when focusing from an infinitely distant object to an object at a shorter distance, the speeds of forward movement of the 1st, 2nd and 3rd lens groups are made progressively slower as indicated by arrows in FIGS. 1 to 3. In this manner stability of aberration correction is maintained throughout the extended range of variation of the image magnification. And, a diaphragm is positioned in a space between the 1st and 2nd lens groups, and the 3rd lens group is constructed as including at least one positive lens, whereby the aberrations are well corrected. In the case where the 3rd lens group is in the form of a singlet lens, it is preferred to use a glass material of low dispersion for the purpose of correcting lateral chromatic aberration when in closeup shooting. To facilitate a further improvement of the lateral chromatic aberration at increased magnifications, it is recommended to construct the 3rd lens group from a plurality of lens elements.

In the present invention, though the above-described form of objective lens enables the image aberrations to be maintained stable throughout an extended range of variation of the image magnification, further assistance in the stabilization of the aberrations can be obtained when the following conditions are satisfied:

    0.12<Δd.sub.1 /ΔS<0.21                         (1)

    0.04<Δd.sub.2 /ΔS<0.21                         (2)

    R.sub.2 <0                                                 (3)

    N1<N2                                                      (4)

    40<νI<44                                                (5)

wherein Δd₁ and Δd₂ are the amounts of variation of the intervals between the 1st and 2nd lens groups and between the 2nd and 3rd lens groups respectively when focusing from an infinitely distant object to close objects; ΔS is the amount of forward movement of the 3rd lens group; R2 is the radius of curvature of the 2nd lens surface counting from the front; N1 and N2 are the indices of refraction of the glasses from which the 1st and 2nd lens elements are made up respectively; and νI is the average Abbe number of the lens elements constituting the 1st lens component.

Among the above-cited conditions, conditions (1) and (2) are the fundamental ones concerning the present invention for good correction of outward coma and spherical aberration when focused to close objects. Condition (3) is to correct negative distortion due to the variation of the image magnification. Conditions (4) and (5) concern the Petzval sum and the achromatism of the 1st lens group.

Condition (1) represents a compromise between the requirements of correcting the outward coma and spherical aberration in the closeup shooting position. As the value of said interval increases, the off-axis ray of light arrives at the 2nd lens group at a higher height of incidence and, therefore, is refracted more strongly by the 2nd lens group to produce inward coma which can be well balanced with the outward coma resulting from the variation of the image magnification. On the other hand, however, the paraxial ray diverges in said space so that as this interval increases, under-correction of spherical aberration results. For this reason, when the upper limit of condition (1) is exceeded, though it is advantageous at the correction of outward coma, the spherical aberration is largely under-corrected. This also causes the focal length to be largely increased. When the lower limit is exceeded, though the range of variation of the spherical aberration is lessened, it is made more difficult to remove the outward coma.

Condition (2) cooperates with condition (1) to contribute a similar effect to the correction of outward coma and the correction of spherical aberration. Since expansion of said interval has a similar advantage to that described in connection with condition (1), its aim is to reduce the deterioration of the various aberrations which would be otherwise caused when the interval Δd₁ is varied in surplus to account for the variation of the interval Δd₂, whereby even at extraordinarily high image magnifications the variation of aberrations is limited to a minimum. If a correction of the outward coma is attemped by imparting independent variation only to, for example, the interval Δd₂ while the interval Δd₁ of condition (1) is left unchanged during focusing, it will result that the required amount of variation of the interval Δd₂ becomes very large, and, therefore, that the under-correction of spherical aberration reaches an unacceptable level. Additional drawbacks wherein the outer diameter of the 3rd lens group is increased, and the focal length is considerably increased will also be involved. When the upper limit of condition (2) is exceeded, under-correction of spherical aberration results, and the focal length is increased. When the lower limit is exceeded, the advantage of correcting the outward coma will be lost.

Condition (3) is to limit the negative distortion in the closeup position to a minimum. When this condition is violated, it becomes difficult in any case to perform good correction of negative distortion.

Condition (4) cooperates with condition (5) to well correct the various aberrations. When this condition is violated, achromatization of the 1st lens group, avoidance of a positive large increase of the Petzval sum and removal of coma are difficult to perform. In particular, in order to limit the range of inclination of the image surface resulting from the variation of the image magnification to a minimum, it is required to take the N2 at a high value and to retain the Petzval sum to a small value.

Condition (5) is to achromatize the 1st lens group. In the present invention, every constituent lens element is made up from a relatively high Abbe number, and the satisfaction of the conditions with regard to achromatism is also taken into account. When the upper limit is exceeded, achromatism within the 1st lens group becomes difficult to achieve. On the other hand, the lower limit represents the average Abbe number of the glasses usable in the 1st lens group in view of the refractive indices of the glasses of the positive and negative lenses.

Three examples of specific objective lenses of the invention can be constructed in accordance with the numerical data given in the following tables for the radii of curvature, R, the axial thicknesses and air separations, D, and the indices of refraction, N, and the Abbe numbers, ν, of the glasses of the lens elements, all expressed with the usual subscripts numbered consecutively from front to rear.

    ______________________________________                                         Embodiment 1                                                                   F = 1 FNO = 1:2 2ω = 45.5                                                ______________________________________                                         R1 = 1.4306                                                                              D1 = 0.0638  N1 = 1.77250                                                                               ν1 = 49.6                                R2 = -5.8384                                                                             D2 = 0.0030                                                          R3 = 0.4423                                                                              D3 = 0.0876  N2 = 1.79952                                                                               ν2 = 42.2                                R4 = 0.8928                                                                              D4 = 0.0155                                                          R5 = 2.4975                                                                              D5 = 0.0597  N3 = 1.69895                                                                               ν3 = 30.1                                R6 = 0.3489                                                                              D6 = Variable                                                        R7 = -0.3137                                                                             D7 = 0.0309  N4 = 1.67270                                                                               ν4 = 32.1                                R8 = -0.8855                                                                             D8 = 0.0633  N5 = 1.77250                                                                               ν5 = 49.6                                R9 = -0.4636                                                                             D9 = 0.0031                                                          R10 = -2.2632                                                                            D10 = 0.0453 N6 = 1.77250                                                                               ν6 = 49.6                                R11 = -0.5787                                                                            D11 = Variable                                                       R12 = -1.6914                                                                            D12 = 0.0329 N7 = 1.56873                                                                               ν7 = 63.1                                R13 = -0.7423                                                                            D13 =  0.0252                                                                               N8 = 1.69895                                                                               ν8 = 30.1                                R14 = -0.9305                                                                  ______________________________________                                         Reproduction                                                                   Ratio            D6      D11                                                   ______________________________________                                         β= 0.0      0.1699  0.0031                                                β= -0.85    0.2993  0.1248                                                ______________________________________                                          ##STR1##                                                                       ##STR2##                                                                 

    ______________________________________                                         Embodiment 2                                                                   F = 1 FNO = 1:2 2ω = 45.5                                                ______________________________________                                         R1 = 1.2261                                                                              D1 = 0.0638  N1 = 1.72600                                                                               ν1 = 53.5                                R2 = -12.3375                                                                            D2 = 0.0030                                                          R3 = 0.4270                                                                              D3 = 0.0736  N2 = 1.83481                                                                               ν2 = 42.7                                R4 = 0.9051                                                                              D4 = 0.0136                                                          R5 = 1.4972                                                                              D5 = 0.0504  N3 = 1.74950                                                                               ν3 = 35.3                                R6 = 0.3177                                                                              D6 = Variable                                                        R7 = -0.3133                                                                             D7 = 0.0310  N4 = 1.64769                                                                               ν4 = 33.8                                R8 = -1.0252                                                                             D8 = 0.0633  N5 = 1.77250                                                                               ν5 = 49.6                                R9 = -0.5022                                                                             D9 = 0.0031                                                          R10 = -1.0510                                                                            D10 = 0.0453 N6 = 1.77250                                                                               ν4 = 49.6                                R11 = -0.5850                                                                            D11 = 0.0031                                                         R12 = -1.3557                                                                            D12 = 0.0388 N7 = 1.69680                                                                               ν7 = 55.5                                R13 = -0.7731                                                                            D13 =  Variable                                                      R14 = -1.5200                                                                            D14 = 0.388  N8 = 1.61800                                                                               ν8 = 63.4                                R15 = -0.6047                                                                            D15 = 0.0213 N9 = 1.84666                                                                               ν9 = 23.9                                R16 = -0.7460                                                                  ______________________________________                                         Reproduction                                                                   Ratio            D6      D13                                                   ______________________________________                                         β = 0.0     0.2524  0.0031                                                β = -0.85   0.4257  0.0464                                                ______________________________________                                          ##STR3##                                                                       ##STR4##                                                                 

    ______________________________________                                         Embodiment 3                                                                   F = 1 FNO = 1:2 2ω= 45.5                                                 ______________________________________                                         R1 = 1.8432                                                                              D1 = 0.0638  N1 = 1.77250                                                                               ν1 = 49.6                                R2 = -4.0060                                                                             D2 = 0.0030                                                          R3 = 0.4332                                                                              D3 = 0.0678  N2 = 1.79952                                                                               ν2 = 42.2                                R4 = 0.9612                                                                              D4 = 0.0294                                                          R5 = 3.3725                                                                              D5 = 0.0484  N3 = 1.69895                                                                               ν3 = 30.1                                R6 = 0.3621                                                                              D6 = Variable                                                        R7 = 0.3308                                                                              D7 = 0.0309  N4 = 1.67270                                                                               ν4 = 32.1                                R8 = -1.4750                                                                             D8 = 0.0633  N5 = 1.77250                                                                               ν5 = 49.6                                R9 = -0.4709                                                                             D9 = 0.0031                                                          R10 = -2.7852                                                                            D10 = 0.0581 N6 = 1.77250                                                                               ν6 = 49.6                                R11 = -0.6416                                                                            D11 = Variable                                                       R12 = -1.7231                                                                            D12 = 0.0543 N7 = 1.56873                                                                               ν7 = 63.1                                R13 = -1.0557                                                                  ______________________________________                                         Reproduction                                                                   Ratio            D6      D11                                                   ______________________________________                                         β= 0.0      0.1659  0.0029                                                31 0.85          0.2652  0.1595                                                ______________________________________                                          ##STR5##                                                                       ##STR6##                                                                 

The various aberrations of the objective in embodiment 1 with an object at infinity and at a reproduction ratio of 0.85 are illustrated in FIGS. 4-a to d and FIGS. 5-a to d respectively.

The various aberrations of the objective in embodiment 2 with an object at infinity and at a reproduction ratio of 0.85 are illustrated in FIGS. 6-a to d and FIGS. 7-a to d respectively.

The various aberrations of the objective in embodiment 3 with an object at infinity and at a reproduction ratio of 0.85 are illustrated in FIGS. 8-a to d and FIGS. 9-a to d respectively.

In the drawings, S designates the sagittal image surface, M the meridional image surface and Y the image height. 

What is claimed is:
 1. A large relative aperture objective lens including:(a) a first component having a positive refractive power; (b) a second component arranged on the image side of said first component and having a positive refractive power; and (c) a third component arranged on the image side of said second component and having a positive refractive power,whereby as focusing is effected down from infinity to shorter object distances, said first, second and third components move axially toward the object, wherein the speed of movement of said first component is faster than that of movement of said second component, and the speed of movement of said second component is faster than that of movement of said third component.
 2. A large relative aperture objective lens according to claim 1, satisfying the following conditions:

    0.12<Δd.sub.1 /ΔS<0.21                         (1)

    0.04<Δd.sub.2 /ΔS<0.21                         (2)

    R.sub.2 <0                                                 (3)

    N1<N2                                                      (4)

    40<ν.sub.I <44                                          (5)

where Δd₁, Δd₂ and ΔS are respectively the amounts of variation of the air separations between the first and second lens components and between the second and third lens components and the amount of forward movement of the third lens component when focusing from an infinitely distant object to close objects; R2 is the radius of curvature of the second lens surface counting from the front; N1 and N2 are the refractive indices of the glasses of the first and second lens elements respectively; and ν_(I) is the average Abbe number of the lens elements constituting the first lens component. 